U.S. patent application number 11/837734 was filed with the patent office on 2008-02-14 for polishing composition for semiconductor wafer, production method thereof, and polishing method.
This patent application is currently assigned to NIPPON CHEMICAL INDUSTRIAL CO., LTD.. Invention is credited to Masahiro IZUMI, Makiko KURODA, Kuniaki MAEJIMA, Shinsuke MIYABE, Hiroaki TANAKA.
Application Number | 20080038996 11/837734 |
Document ID | / |
Family ID | 39051368 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080038996 |
Kind Code |
A1 |
MAEJIMA; Kuniaki ; et
al. |
February 14, 2008 |
POLISHING COMPOSITION FOR SEMICONDUCTOR WAFER, PRODUCTION METHOD
THEREOF, AND POLISHING METHOD
Abstract
A polishing composition for semiconductor wafers containing
colloidal silica is disclosed, wherein the colloidal silica is
prepared from an active silicic acid aqueous solution obtained by
removing alkali from an alkali silicate aqueous solution and a
quaternary ammonium base, and is stabilized with a quaternary
ammonium base. The polishing composition contains no alkali metals.
The polishing composition contains a buffer solution that is a
combination of a weak acid having a pKa from 8.0 to 12.5 at
25.degree. C. (pKa is a logarithm of the reciprocal of acid
dissociation constant) and a quaternary ammonium base, and exhibits
a buffer action in the range from pH8 to pH11.
Inventors: |
MAEJIMA; Kuniaki; (Tokyo,
JP) ; MIYABE; Shinsuke; (Tokyo, JP) ; IZUMI;
Masahiro; (Tokyo, JP) ; TANAKA; Hiroaki;
(Kanagawa, JP) ; KURODA; Makiko; (Kanagawa,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
NIPPON CHEMICAL INDUSTRIAL CO.,
LTD.
Tokyo
JP
SPEEDFAM CO., LTD.
Kanagawa
JP
|
Family ID: |
39051368 |
Appl. No.: |
11/837734 |
Filed: |
August 13, 2007 |
Current U.S.
Class: |
451/37 ;
257/E21.23; 257/E21.237; 51/308 |
Current CPC
Class: |
C09K 3/1463 20130101;
C09G 1/02 20130101; H01L 21/30625 20130101 |
Class at
Publication: |
451/37 ;
51/308 |
International
Class: |
B24B 7/00 20060101
B24B007/00; C09K 3/14 20060101 C09K003/14 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 14, 2006 |
JP |
2006-220897 |
Claims
1. A polishing composition for a semiconductor wafer comprising: a
colloidal silica being stabilized with a quaternary ammonium base
that is prepared from a quaternary ammonium base and an active
silicic acid aqueous solution obtained by removing alkali from an
alkali silicate aqueous solution; and a buffer solution which is a
combination of a weak acid having a pKa that is a logarithm of a
reciprocal of acid dissociation constant of from 8.0 to 12.5 at
25.degree. C. and a quaternary ammonium base, wherein the polishing
composition contains substantially no alkali metals, and has a
buffer action at 25.degree. C. in the range from pH8 to pH11.
2. The polishing composition for a semiconductor wafer according to
claim 1, wherein the colloidal silica stabilized with a quaternary
ammonium base contains non-spherical silica particles.
3. The polishing composition for a semiconductor wafer according to
claim 1, wherein the polishing composition is a water dispersion
having a silica concentration from 2 to 50% by weight with respect
to a total amount of a colloidal solution.
4. The polishing composition for a semiconductor wafer according to
claim 1, having a conductivity at 25.degree. C. of 15 mS/m or more
based on 1% by weight of silica particles.
5. The polishing composition for a semiconductor wafer according to
claim 4, wherein the polishing composition has a salt of a strong
acid and a quaternary ammonium base so as to adjust the
conductivity at 25.degree. C. of 15 mS/m or more based on 1% by
weight of silica particles.
6. The polishing composition for a semiconductor wafer according to
claim 5, wherein the salt of a strong acid and a quaternary
ammonium base is a quaternary ammonium sulfate, a quaternary
ammonium nitrate, or a quaternary ammonium fluoride.
7. The polishing composition for a semiconductor wafer according to
claim 1, wherein an anion constituting the weak acid is a carbonate
ion and/or a hydrogen carbonate ion; and the quaternary ammonium
base is a choline ion, a tetramethylammonium ion or a
tetraethylammonium ion, or a mixture thereof.
8. The polishing composition for a semiconductor wafer according to
claim 1, wherein an average diameter of silica particles of the
colloidal silica by a BET method is from 10 to 200 nm.
9. A method for producing the polishing composition for a
semiconductor wafer according to claim 1, comprising the steps of:
preparing the active silicic acid aqueous solution by contacting
dilute sodium silicate with a cation exchange resin to remove
sodium ions; allowing to grow colloidal particles by adding the
quaternary ammonium base to adjust the pH in the range from 8 to 11
followed by heating the active silicic acid aqueous solution;
preparing the colloidal silica which is free of alkali metals and
has a silica concentration from 10 to 60 wt % by concentrating
silica with ultrafiltration; and adding the weak acid and the
quaternary ammonium base to the colloidal silica to make a buffer
composition and to adjust the silica concentration in the range
from 2 to 50 wt %.
10. A polishing method comprising: rotating a rotatable polishing
plate and/or a semiconductor wafer while the polishing composition
according to claim 1 is supplied to the polishing plate to polish a
surface of the semiconductor wafer using the polishing plate having
a polishing cloth fixed to both or either of its upper and lower
face, under a state of pressing the semiconductor wafer against the
polishing plate.
11. A polishing method comprising: polishing an edge of a
semiconductor wafer using a drum-shaped polishing member having a
polishing cloth fixed to its surface or with a polishing apparatus
having a polishing member with an arc-shaped working surface,
wherein the polishing member and/or a semiconductor wafer are
rotated while the polishing composition according to claim 1 is
supplied to the polishing member, under a state of the edge of the
semiconductor wafer being pressed against the polishing member.
Description
TECHNICAL FIELD
[0001] The present invention relates to a polishing composition for
semiconductor wafers and a production method thereof. More
specifically, the present invention relates to a polishing
composition for semiconductor wafers used to polish the surface or
edge of a semiconductor wafer, and a production method thereof.
Furthermore, the present invention relates to a polishing method
for allowing the surface and edge of a semiconductor wafer to have
a mirror surface by using the polishing composition for
semiconductor wafers. The semiconductor wafer that is subject to
the polishing of the present invention includes preferably a
silicon wafer and a semiconductor device substrate having a metal
film, an oxide film, a nitride film, or others (hereinafter, called
as metal film and others) formed on its surface.
BACKGROUND ART
[0002] Electronic components such as ICs, LSIs and super LSIs using
semiconductor materials such as single crystal silicon as raw
material, are produced as follows: a single crystal ingot of
silicon or the other compound semiconductors is sliced into thin
disc wafers; a number of fine electronic circuits are built in the
wafer; and then the wafer is broken up into small platelets of
semiconductor element chips. The wafer that is produced by slicing
the ingot is processed into a mirror surface wafer with a
mirror-polished surface and edge through the steps of lapping,
etching, and further polishing. After that, in a
device-manufacturing step, fine electronic circuits are formed on
the surface of the mirror-polished wafer. At present, from the
viewpoint of developing high-speed LSIs, the process for forming
the electronic circuits has been shifting to a new process.
Specifically, in place of a conventional wiring material of Al, Cu
having still lower electrical resistance than Al is used. A low
dielectric film having a still lower dielectric constant than that
of a silicon oxide film is used as an insulating film between
wirings. Further, between Cu and the low dielectric film, a barrier
layer of tantalum or tantalum nitride is interposed so as to
prevent Cu from diffusing into the low dielectric film. In order to
develop such wiring structure and high integration, polishing step
is repeated many times in the process such as (a) planarizing
interlayer insulation films, (b) forming metal interconnections
(plugs) connecting the upper and lower of multilayer wirings, and
(c) forming embedded wirings. In the polishing step, generally, a
semiconductor wafer is put on a surface table having a polishing
cloth of a synthetic resin foam, suede-like synthetic leather or
the like extended and stretched thereon; while the semiconductor
wafer is pressed against the surface table and rotated, a given
amount of polishing composition solution is supplied so as to
polish the semiconductor wafer.
[0003] At the edge of the semiconductor wafer, the above-mentioned
metal film and others are unevenly deposited. Before broken up into
semiconductor element chips, the wafer is supported at the edge
when it is subject to a transportation step or the like, while
keeping the initial disk shape. In the case where the edge of the
wafer is unevenly structural shape at the transportation, micro
cracks of the wafer is caused when the wafer comes into contact
with a transporter, thereby developing fine particles sometimes.
The fine particles developed are scattered in the subsequent steps,
contaminating the finely-processed faces, largely influencing the
yield and quality of products. To prevent the contamination caused
by the fine particles, the edge of the semiconductor wafer is
needed to have mirror-polishing after the metal and other films are
deposited.
[0004] The edge is polished as follows: the edge of the
semiconductor wafer is pressed against the face of a polishing
member having a polishing cloth made of a synthetic resin foam,
synthetic leather, nonwoven fabric or the like applied to the
surface of a polishing cloth support; and then either of the
polishing member and wafer is rotated while a polishing composition
containing a polishing abrasive particles such as silica as a main
ingredient is supplied. As the abrasive particles used here for the
polishing composition, there has been proposed colloidal silica
similar to the one used for silicon wafer edge polishing, fumed
silica, ceria or alumina that is used for polishing of device
wafer, and the like. Particularly, the colloidal silica and fumed
silica have become a focus of attention because they are so fine
that a flat mirror face can be easily formed. The polishing
composition as mentioned above is called also as "slurry", which
may be called as such in some cases below.
[0005] The polishing composition containing the silica abrasive
particles as a main ingredient is given as a solution that contains
alkali components in general. The polishing mechanism can be
described by the combination of chemical action by the alkali
components, specifically chemical corrosion against the surface of
silicon oxide film, metal films or the like, and mechanical
abrasion by the silica abrasive particles. More specifically, the
corrosion by the alkali components produces a thin and soft
corrosion layer on the surface of an object product to be polished
such as a wafer. It is estimated that the resulting corrosion layer
is removed by the mechanical abrasion of the fine abrasive
particles. It is considered that polishing may proceed by repeating
these steps. After polishing, the silica abrasive particles and
alkali components are removed from the surface and edge polished in
a cleaning step.
[0006] A problem that the abrasive particles remain on the wafer
surface in the cleaning step has been pointed out. It is possible
to largely improve such a state that the abrasive particles remain
on the wafer surface by selecting properly polishing conditions or
cleaning processes. On the other hand, polishing speed is largely
lowered and the cleaning process becomes complicated. The problem
has not yet been solved.
[0007] Furthermore, fine-line processing for device wiring has
become more pronounced year by year. According to International
Technology Roadmap for Semiconductors, the aimed figures of the
line width for device wiring are 90 nm for the year of 2004, 65 nm
for 2007, 50 nm for 2010, and 35 nm for 2013. As the line width of
device wiring becomes finer, the semiconductor wafer surface after
polishing is required to have still higher cleanness. The abrasive
used for polishing the semiconductor wafer contains abrasive
particles having a particle diameter of dozens of nanometers as
mentioned above. So far, the diameter of the abrasive particles has
been sufficiently smaller as compared with the line width, so that
the abrasive particles remained on the semiconductor wafer surface
have not posed a large problem. However, with the advancement of
finer-line device wiring, the diameter of the abrasive particles
has become almost the same as the line width of device wiring, and
thus the abrasive particles remained on the semiconductor wafer
surface have lead to malfunction of devices. This poses a serious
problem.
[0008] Conventionally, various polishing compositions have been
proposed for mirror polishing of semiconductor wafers. For example,
U.S. Pat. No. 4,671,851 discloses colloidal silica containing
sodium carbonate and an oxidizing agent. EP0357205A1 discloses
colloidal silica containing ethylenediamine. JP11-60232A discloses
silica particles having a shape of cocoon. JP6-53313 discloses a
method for polishing device wafers using an aqueous solution
containing ethylenediamine pyrocatechol and silica fine powder.
JP8-83780 discloses a method for polishing semiconductor wafers
using an aqueous solution containing glycine, hydrogen peroxide,
benzotriazole, and silica fine powder. U.S. Pat. No. 5,904,159
discloses an abrasive obtained by dispersing fumed silica having an
average diameter from 5 to 30 nm in a KOH aqueous solution, and a
method for producing the abrasive. U.S. Pat. No. 5,230,833A
discloses polishing slurry of colloidal silica from which sodium is
removed by cation exchange. The addition of an amine as a polishing
promoter into the polishing slurry is proposed, and also the
addition of a quaternary ammonium salt as a bactericide is
proposed. JP2002-105440 discloses the use of a specific amine.
JP2003-89786 discloses high-purity colloidal silica for polishing
that is prepared using tetramethylammonium hydroxide in place of
sodium hydroxide as an alkali agent used in the step of growing
colloidal silica particles, and is substantially free of sodium.
U.S. Pat. No. 6,300,249B1 discloses a silicon oxide colloid
solution that is prepared as a buffer solution having a buffering
action in the range of pH8.7 to pH 10.6 by adding any one of
combinations selected from weak acid and strong base, and weak acid
and weak base. U.S. Pat. No. 6,238,272B1 discloses a polishing
composition admixed with an alkali component and an acid component
to have a buffering action, and using quaternary ammonium as the
alkali component.
[0009] When colloidal silica is used in a manner as disclosed in
U.S. Pat. No. 4,671,851 and EP0357205A1, there is a problem of
impurity. Because sodium silicate is used as raw material for the
production of the colloidal silica, relatively large amounts of
alkali metals such as sodium are incorporated in the colloidal
silica obtained. Therefore, the abrasive particles of the colloidal
silica tend to be left behind after polishing. The silica particles
having the shape of cocoon as disclosed in JP11-60232A have high
purity and are excellent in terms of containing no alkali metals,
since the silica particles are prepared using an organic silicon
compound as raw material. However, these silica particles are soft
and have a disadvantage of slow polishing speed. The methods
disclosed in JP6-53313A and JP8-83780A are excellent in terms of
containing no alkali metals. However, fumed silica is considered to
be used because these documents describe that silica fine powder is
used. The filmed silica can bring a high polishing speed, but may
be easy to develop scratches on the polished face. U.S. Pat. No.
5,904,159A discloses fumed silica slurry, which may provide a high
polishing speed, but easily develop scratches. In addition, a KOH
aqueous solution is used for the slurry, so that the slurry is not
appropriate as a polishing material. The low-sodium content
colloidal silica described in U.S. Pat. No. 5,230,833A is admixed
with an amine as a polishing promoter and a little amount of a
quaternary ammonium salt as a disinfectant having a polishing
promoting effect as well. In the example, as the amine,
aminoethylethanolamine and piperazine are disclosed. Recent years,
it has been found that amine causes metal contamination in wafers,
particularly copper contamination owing to the metal chelating
action of amine. Further, U.S. Pat. No. 5,230,833A describes that
KOH is used for pH control, so that the problem to be solved is the
reduction of sodium content. JP2002-105440A describes the risks of
wafer contamination caused by aminoethylethanolamine. The colloidal
silica described in JP2003-89786A contains no sodium in the water
phase and the surface and the inside of the particles, and,
therefore, it is an extremely desirable abrasive. However, pH
fluctuation during polishing is large when using quaternary
ammonium hydroxide alone, and also the pH may be lowered largely by
the atmospheric carbon dioxide, and thus a stable polishing speed
may not be attained.
[0010] If comparisons are made with edge polishing and surface
polishing of semiconductor wafers, the former has a shorter time of
contacting a polishing cloth to the polishing surface than the
latter, so that the pressure applied to the polishing surface of
the edge is made to be higher and the linear velocity of the
polishing cloth with respect to the polishing surface is made to be
higher. Namely, the edge is polished under extremely harsh
conditions as compared with the surface. The edge of semiconductor
wafers has an extremely large surface roughness. Under such process
conditions, conventional compositions containing fumed silica for
polishing the surface of semiconductor wafers hardly provide
sufficient polishing speed and surface roughness.
SUMMARY OF THE INVENTION
[0011] A first invention of the present invention provides a
polishing composition for a semiconductor wafer comprising: a
colloidal silica being prepared from an active silicic acid aqueous
solution obtained by removing alkali from an alkali silicate
aqueous solution, and a quaternary ammonium base, and being
stabilized with a quaternary ammonium base; and a buffer solution
which is a combination of a weak acid having a pKa that is a
logarithm of a reciprocal of acid dissociation constant of from 8.0
to 12.5 at 25.degree. C. and a quaternary ammonium base, wherein
the polishing composition contains substantially no alkali metals,
and has a buffer action at 25.degree. C. in the range from pH8 to
pH11.
[0012] The colloidal silica stabilized with a quaternary ammonium
base preferably contains non-spherical silica particles.
[0013] The polishing composition is preferably a water dispersion
having a silica concentration from 2 to 50% by weight with respect
to a total amount of a colloidal solution.
[0014] Further, the polishing composition preferably has a
conductivity at 25.degree. C. of 15 mS/m or more based on 1% by
weight of silica particles.
[0015] The polishing composition preferably has a salt of a strong
acid and a quaternary ammonium base so as to adjust the
conductivity at 25.degree. C. of 15 mS/m or more based on 1% by
weight of silica particles.
[0016] The salt of a strong acid and a quaternary ammonium base is
preferably a quaternary ammonium sulfate, a quaternary ammonium
nitrate, or a quaternary ammonium fluoride.
[0017] The anion constituting the aforementioned weak acid is
preferably a carbonate ion and/or a hydrogen carbonate ion, and the
quaternary ammonium base is preferably a choline ion, a
tetramethylammonium ion or a tetraethylammonium ion, or a mixture
thereof. "Choline" is a popular name of trimethyl (hydroxyethyl)
ammonium.
[0018] Further, an average diameter of silica particles of the
colloidal silica in the polishing composition for a semiconductor
wafer by a BET is preferably from 10 nm to 200 nm.
[0019] A second invention of the present invention provides a
method for producing the polishing composition for a semiconductor
wafer described above, comprising the steps of: preparing the
active silicic acid aqueous solution by contacting dilute sodium
silicate with a cation exchange resin to remove sodium ions;
allowing to grow colloidal particles by adding the quaternary
ammonium base to adjust the pH in the range from 8 to 11 followed
by heating the active silicic acid aqueous solution; preparing the
colloidal silica which is free of alkali metals and has a silica
concentration from 10 to 60 wt % by concentrating silica with
ultrafiltration; and adding the weak acid and the quaternary
ammonium base to the colloidal silica to make a buffer composition
and to adjust the silica concentration in the range from 2 to 50 wt
%.
[0020] A third invention of the present invention provides a
polishing method comprising: rotating a rotatable surface table
and/or a semiconductor wafer while the polishing composition
described above is supplied to the surface table to polish a
surface of the semiconductor wafer using the surface table having a
polishing cloth fixed to both or either of its upper and lower
face, under a state of the semiconductor wafer being pressed
against the surface table.
[0021] A fourth invention of the present invention provides a
polishing method comprising: polishing an edge of a semiconductor
wafer using a drum-shaped polishing member having a polishing cloth
fixed to its surface or with a polishing apparatus having a
polishing member with an arc-shaped working surface, wherein the
polishing member and/or a semiconductor wafer are rotated while the
polishing composition described above is supplied to the polishing
member, under a state of the edge of the semiconductor wafer being
pressed against the polishing member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 shows a TEM image of colloidal silica of the present
invention obtained in the examples.
[0023] FIG. 2 shows a TEM image of colloidal silica used in the
comparative example.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention is to provide a polishing composition
that suppresses remaining abrasive particles on the surface of a
semiconductor wafer, keeps a high polishing speed, and allows the
surface and edge of the semiconductor wafer to attain a mirror
surface with a good roughness, and a production method thereof.
Further, the present invention is to provide a polishing method for
allowing the surface and edge of a semiconductor wafer to have a
mirror surface, using the polishing composition.
[0025] The present inventors have found that the surface and edge
of a semiconductor wafer can be mirror-polished effectively by
using a polishing composition for a semiconductor wafer comprising
a colloidal silica stabilized with a quaternary ammonium base, and
a buffer solution which is a combination of a weak acid having a
pKa that is a logarithm of a reciprocal of acid dissociation
constant of from 8.0 to 12.5 at 25.degree. C. and a quaternary
ammonium base, wherein the polishing composition contains
substantially no alkali metals, and has a buffer action at
25.degree. C. in the range from pH8 to pH11. Thus, the present
invention has been accomplished based on this finding.
[0026] The polishing composition of the present invention provides
such a remarkable effect that particle contamination, particularly
remains of abrasive particles (hereinafter, described as "abrasive
remains") on a surface portion in polishing a semiconductor wafer
and the like is not easy to take place. "Abrasive remains"
represents such a state that abrasive particles of a polishing
composition adhere to a surface portion of a wafer during
polishing, and the abrasive particles are left behind on the
surface portion of the wafer even after cleaning. The present
invention overcomes the problem of abrasive remains at the surface
portion, for which countermeasures have been comparatively
insufficiently taken so far. The present invention provides a
polishing composition that exhibits an excellent polishing
performance for mirror-polishing of wafers and a stability of the
polishing performance. In this way, the present invention provides
an extremely large effect on the related technology fields.
[0027] It is quite important that the polishing composition of the
present invention contains substantially no alkali metals in the
water phase and the surface and the inside of silica particles, and
is composed of colloidal silica stabilized with a quaternary
ammonium base. Commercially-available colloidal silica stabilized
with sodium contains generally 20 to 50 wt % of silica (SiO.sub.2)
and 0.1 to 0.3 wt % of Na.sub.2O (0.07 to 0.22 wt % in terms of
Na). The content of sodium is from 0.2 to 0.7 wt % expressed in
terms of silica. Generally, colloidal silica having a larger
particle diameter has lower sodium content.
[0028] Here, there is briefly mentioned about "stabilization". For
example, silica particles dispersed in pure water have silanol
groups on their surface, and the outside of the particles is
surrounded only by water molecules. The particles vibrate and move
by Brownian motion, so that they collide with one another and are
linked together through dehydration bonding between the silanol
groups. As the linkage expands, the colloid has increased
viscosity, and eventually turns into a gel. For example, silica
particles dispersed in a dilute sodium hydroxide aqueous solution
at about pH9 have silanol groups on their surface, and there exist
hydrated sodium cations outside of the silanol groups. Thus, the
particles are charged in anionic. On the outside of the hydrated
phase of the sodium cations, there exist OH.sup.- ions closely, and
further on the outside thereof there exist water molecules. The
silica particle surface having such a restraint phase as described
above provides repulsion among the particles, so that collision and
linking among particles is inhibited. This state is called as
"stabilization".
[0029] The colloidal silica stabilized with sodium hydroxide
contains sodium in the water phase and the surface and the inside
of the silica particles. The content of sodium inside of the silica
particles is from 0.1 to 0.5 wt % based on silica. The sodium
contained in the water phase and the surface of the silica
particles can be removed by contacting the colloidal silica with a
proton-type cation exchange resin. However, the sodium inside of
the silica particles partly migrates to the surface of the
particles by degrees taking several-months period at normal
temperature, and this migration may be detected as a pH change. As
a result, the water phase and surface of the silica particles hold
sodium again.
[0030] The colloidal silica stabilized with a quaternary ammonium
base as in the present invention contains sodium in very small
quantity. As mentioned later, in the preferred production method of
the present invention, sodium silicate is contacted with a cation
exchange resin to remove sodium ion thereby to prepare an active
silicic acid aqueous solution. However, sodium ion is not
completely removed, and the resulting active silicic acid aqueous
solution contains sodium ion in very small quantity. Generally, the
amount of the sodium ion is 50 ppm or less by weight based on
silica. Such a small amount of sodium is acceptable in the present
invention. In the present invention, "substantially contains no
alkali metals" is used in this meaning.
[0031] The present inventors are the first who have found that
silica particles of colloidal silica stabilized with a quaternary
ammonium base wherein sodium in the water phase and the surface of
the silica particles is removed are not easy to adhere to the wafer
surface. The mechanism can be speculated as follows. In the case of
the colloidal silica stabilized with sodium hydroxide, water is
slightly evaporated during the passage of a little time while
polishing slurry is on the surface of a wafer after polishing,
whereby sodium hydroxide corrodes the silica particles and the
metal (or metal oxide) surface of the wafer, and the silica
particles and metal hydroxide are bonded together. It is considered
that the bonding may be caused by fusing the silica particle
surface and metal hydroxide surface or by electrostatic interaction
between the minus charge of the silica and the plus charge of the
metal hydroxide surface.
[0032] On the other hand, in the case of the colloidal silica
stabilized with a quaternary ammonium base, quaternary ammonium
ions exist on the surface of silica particles and on the surface of
a wafer as well. On both surfaces, the alkyl groups of the
quaternary ammonium ions are exposed to the outside. The repulsion
among these alkyl groups prevents the silica particles from
adhering to the wafer surface. In the field of metal corrosion
inhibition, quaternary ammonium bases and amines are used as an
inhibitor (corrosion inhibitor). The nitrogen atom of the inhibitor
molecule adsorbs to the metal surface, and the alkyl group of the
molecule direct to the liquid phase so as to form a water-repellent
phase on the metal surface, which is considered to exert corrosion
inhibition effect. Similar corrosion inhibition effect may be
considered to exert on the wafer surface.
[0033] The quaternary ammonium base is preferably, for example,
choline ion, tetramethylammonium ion, tetraethylammonium ion, or a
mixture thereof. The other preferable quaternary ammonium base may
include a quaternary ammonium ion composed of an alkyl group having
4 or less carbon atoms or a hydroxyalkyl group having 4 or less
carbon atoms. The alkyl group may include, for example, a methyl
group, an ethyl group, a propyl group and a butyl group. The
hydroxyalkyl group may include, for example, a hydroxymethyl group,
a hydroxyethyl group, a hydroxypropyl group and a hydroxybutyl
group. Specifically, tetrapropylammonium ion, tetrabutylammonium
ion, methyltrihydroxyethylammonium ion, triethyl (hydroxyethyl)
ammonium ion or the like is preferable, because they are easily
available.
[0034] Further, the other preferable quaternary ammonium bases may
also include benzyltrimethylammonium ion, phenyltrimethylammonium
ion and the like, which are also easily available.
[0035] Depending on their organic groups, quaternary ammonium bases
change their performances of corrosion and polishing against wafers
and also change their cleaning performance of the abrasive
particles. Therefore, preferably these may be appropriately
selected on use. Two or more of them may be used preferably in
combination.
[0036] The polishing composition of the present invention
preferably keeps its pH within a range from 8 to 11 at 25.degree.
C. as a whole composition in order to maintain the stable polishing
performance at the actual polishing process. At a pH lower than 8,
polishing speed is lowered and sometimes goes out of the practical
range. At a pH is higher than 11, etching rate tends to become too
high at portions other than the polishing portions. In addition,
silica particles possibly start to aggregate and, thereby the
stability of the polishing composition becomes lowered and
sometimes goes out of the practical range.
[0037] It is preferable that the pH as a whole composition is not
easy to fluctuate by possible external conditions such as friction,
heat, contact to the outside air, and mixing with the other
components. Particularly in the case of polishing the edge of a
semiconductor wafer, the polishing composition is preferably
circulated on use. That is, the polishing composition supplied to
the polishing portions from a slurry tank is circulated back to the
slurry tank. The polishing composition containing only alkaline
agents lowers its pH in a short period of time on use because of
the dissolution of the portions to be polished or mixing of
cleaning water. The variation of the polishing speed caused by the
pH fluctuation possibly results in poor polishing or adversely
results in overpolishing caused by excessive polishing.
[0038] In order to keep the polishing composition of the present
invention at a constant pH, the polishing composition of the
present composition preferably has a buffer solution composition
that is given by a combination of a weak acid having a pKa from 8.0
to 12.5 at 25.degree. C. (pKa is a logarithm of the reciprocal of
acid dissociation constant) and a quaternary ammonium strong base.
In this case, it is preferred that the buffer action exert in the
range from pH8 to pH11 at 25.degree. C. The buffer action within
the range from pH8 to pH11 means that the pH of the polishing
composition of the present invention is in the range from pH8 to
pH11 after the composition is diluted 100 times with water.
[0039] The anion that forms the weak acid in the buffer solution is
preferably carbonate ion and/or hydrogen carbonate ion. In addition
to that, the cation that forms the quaternary ammonium strong base
is preferably at least one kind selected from choline ion,
tetramethylammonium ion and tetraethylammonium ion. The other
quaternary ammonium ion described above may also be used.
[0040] In the present invention, it is preferred that the polishing
composition itself is a solution with a strong buffer action that
has a little change in pH against fluctuation in the outside
conditions. The buffer solution may be prepared by using a weak
acid having a pKa from 8.0 to 12.5 at 25.degree. C. (pKa is a
logarithm of the reciprocal of acid dissociation constant, Ka) and
a quaternary ammonium strong base in combination as described
above. When the logarithm (pKa) of the reciprocal of the acid
dissociation constant at 25.degree. C. is less than 8.0, a large
quantity of a weak acid and a strong base is undesirably required
to elevate the pH. When the logarithm (pKa) of the reciprocal of
the acid dissociation constant at 25.degree. C. is larger than
12.5, a buffer solution having a stable and strong buffer action in
the range from pH8 to pH11 is not easily formed and thus
undesirable.
[0041] In the present invention, as the weak acid used to prepare
the polishing composition having buffer action, there may be
mentioned preferably, for example, carbonic acid (pKa=6.35 and
10.33). Besides these, there may be mentioned boric acid
(pKa=9.24), phosphoric acid (pKa=2.15, 7.20 and 12.35), a
water-soluble organic acid, and others. A mixture of these acids
may also be used. As the strong base, there may be used a
quaternary ammonium base hydroxide. The buffer solution of the
present invention is a solution given by the combination described
above. In the buffer solution, the weak acid is dissociated into
ions having different valences, or the weak acid exists both in
dissociated and undissociated states. The buffer solution has such
a characteristic property that the pH changes by only a little when
a small amount of acid or base is mixed.
[0042] In the present invention, polishing speed can be remarkably
improved by increasing the conductivity of the polishing
composition. The conductivity is a measure of an ability to conduct
an electric current through a liquid and is represented by the
reciprocal of electrical resistivity. In the present invention, the
conductivity is represented by converting the value (milli-Siemens)
into the value based on 1 wt % of silica. In the present invention,
a conductivity of 15 mS/m1%-SiO.sub.2 or more at 25.degree. C. is
preferable for improving the polishing speed, and a conductivity of
20 mS/m/1%-SiO.sub.2 or more is especially preferable. The upper
limit of the conductivity differs depending on the diameter of the
silica particles, but is around 60 mS/m/1%-SiO.sub.2.
[0043] The polishing process using the polishing composition of the
present composition involves application of the chemical action of
the alkali components of the polishing composition, specifically
the corrosive property of the alkali components against the product
to be polished including a silicon oxide film and a metal film.
Namely, owing to the corrosive property of alkalis, a corroded thin
layer is formed on the surface of the product to be polished such
as a wafer. The process proceeds by removing the thin layer with
the mechanical action of the fine abrasive particles. The corrosion
of the metal film is an oxidation reaction of metal: that is,
electrons are transferred to the metal surface from the solution in
contact with the metal surface; and the metal is dissolved into the
solution in the form of a metal hydroxide ion. In order to allow
the electrons to transfer swiftly, it is desirable that the
conductivity of the solution be high.
[0044] There may be two methods for improving the conductivity. In
one method, the concentration of the buffer solution is increased.
In the other method, salts are added. These two methods may be used
in combination.
[0045] The concentration of the buffer solution may be increased by
increasing only the concentrations of the acid and base while
keeping their molar ratio unchanged.
[0046] The salts used in the method of adding salts are composed of
a combination of acid and base. The addition of salts lowers the
stability of the colloid, so that there may be a limitation on the
addition. Any acid may be used, including a strong acid and a weak
acid. A mineral acid, an organic acid, or a mixture thereof may be
used. As the base, there may be used preferably a water-soluble
quaternary ammonium base hydroxide. The addition of a salt of weak
acid and strong base, a salt of strong acid and weak base, or a
salt of weak acid and weak base may possibly is change the pH of
the buffer solution, so that the addition in large quantity is not
desirable.
[0047] The salt of strong acid and quaternary ammonium base is
preferably at least one kind selected from quaternary ammonium
sulfate, quaternary ammonium nitrate, and quaternary ammonium
fluoride. The cation that forms the quaternary ammonium strong base
is preferably at least one kind selected from choline ion,
tetramethylammonium ion, or tetraethylammonium ion. As the other
quaternary ammonium ion, there may be used the one described
above.
[0048] In the polishing composition of the present invention, the
silica particles of the colloidal silica have a BET average
diameter from 10 to 200 nm, and particularly preferably from 10 to
120 nm. The BET average diameter is an average primary diameter of
particles that is obtained as follows: the specific surface area of
the powdered colloidal silica is measured by the BET method with
N.sub.2-gas adsorption; and then the average primary diameter on
the assumption that the particles are spherical is calculated based
on the following equation from the specific surface area.
[0049] 2720/specific surface area (m.sup.2/g)=average primary
diameter (nm) of particles on the assumption that the particles are
spherical.
[0050] It is also desirable that the polishing composition of the
present invention contains a chelating agent capable of forming a
water-insoluble chelate compound of copper. The preferable
chelating agent may include, for example, azoles such as
benzotriazole and quinoline derivatives such as quinolinol and
quinaldic acid. As described above, a chelating agent such as
ethanol amine that forms a water-soluble chelate compound of copper
is not desirable.
[0051] The polishing composition of the present invention may be
admixed with a surfactant, a dispersant, a defoaming agent, an
anti-sediment agent, and others so as to improve the properties
thereof. As the surfactant, dispersant, defoaming agent, and
anti-sediment agent, there may be mentioned water-soluble organic
substances, inorganic layered compounds, and others. Although the
polishing composition of the present invention is an aqueous
solution, there may be admixed with an organic solvent. The
polishing composition of the present invention may be used by
admixing with the other abrasives such as colloidal alumina,
colloidal ceria, and colloidal zirconia, bases, additives, water,
and others when polishing is performed.
[0052] In the subsequent description, the production method of the
polishing composition according to the present invention containing
colloidal silica stabilized with a quaternary ammonium base will be
mentioned. Firstly, as an alkali silicate aqueous solution used as
raw material, a sodium silicate aqueous solution, conventionally
called as water glass (JIS No. 1 to 4 water glass or the like), is
preferably used. The water glass is relatively inexpensive and
easily available. Considering polishing semiconductor products
incompatible with sodium ions, a potassium silicate aqueous
solution is also suitable for the raw material. The alkali silicate
aqueous solution may be prepared also by dissolving solid alkali
metasilicate in water Alkali metasilicate is produced by way of
crystallization, so that an alkali silicate with reduced impurities
is available. The alkali silicate aqueous solution may be diluted
with water if necessary.
[0053] An alkali silicate aqueous solution diluted with water is
contacted with a cation exchange resin to prepare an active silicic
acid aqueous solution. As the cation exchange resin used in the
present invention, there may be selected a known resin as
appropriate, but there is no particular limitation. The contacting
process of the alkali silicate aqueous solution and the cation
exchange resin is, for example, as follows: an alkali silicate
aqueous solution is diluted with water to obtain a solution having
a silica concentration from 3 to 10 wt %; the solution is contacted
with a H-type strong acid cation exchange resin to be dealkalized;
and then, if necessary, the solution is contacted with an OH-type
strong basic anion exchange resin to be deanionized. In this way,
an active silicic acid aqueous solution is prepared. Various
proposals have made on the details of the contacting conditions so
far. Any disclosed conditions may be adaptable to the present
invention.
[0054] Then, a process for allowing to grow colloid particles is
performed. In this growing process, a quaternary ammonium base is
employed in place of an alkali metal hydroxide conventionally used.
As the quaternary ammonium base, there may be used the ones
described above. The growing process proceeds in accordance with
conventional processes: for example, in order to allowing to grow
the colloid particles, a quaternary ammonium base is added to the
active silicic acid aqueous solution to adjust the pH at 8 to 11 at
25.degree. C., and then the temperature is elevated to 60 to
240.degree. C. In the case where the temperature is elevated to
100.degree. C. or more, hydrothermal treatment using an autoclave
is employed. The higher the temperature is, the larger the diameter
of the particles obtained is. In addition, a buildup process may be
employed. That is, a quaternary ammonium base is added to a part of
the active silicic acid aqueous solution to adjust the pH within 8
to 11 at 25.degree. C., the temperature is elevated to 60 to
240.degree. C. to form seed sol, and then the residual active
silicic acid is added to the seed sol. The build-up process is
generally performed at 80 to 100.degree. C. under an atmospheric
pressure. In either of these methods employed, the growing process
is performed so as to result in the silica particles being grown
into 10 to 200 nm in diameter The dispersion state of the particles
may be mono-dispersed or secondary aggregated. The dispersion state
of the particles may be selected as appropriate depending on
applications. The particles may be spherical or non-spherical. The
shape of the particles may be selected as appropriate depending on
applications. As opposed to conventional production methods using
alkali metal hydroxide, non-spherical particles can be easily
produced by the particle-growing process using a quaternary
ammonium base.
[0055] Next, the resulting silica is concentrated. Evaporative
concentration of water may be employed, but concentration by
ultrafiltration is more advantageous in terms of energy
efficiency.
[0056] An ultrafiltration membrane used for concentrating silica by
ultrafiltration is explained. Separation using the ultrafiltration
membrane is applied to the particles having a diameter from 1 nm to
several microns. The ultrafiltration membrane is also applied to
the separation of dissolved polymer substances. When the size of
the particles to be separated is in the order of nanometers,
filtration accuracy is represented in terms of fractionation
molecular weight. In the present invention, an ultrafiltration
membrane having a fractionation molecular weight of 15,000 or less
may be suitably used. With a membrane specified in the above range,
particles having a diameter of 1 nm or more can be separated. More
preferably, an ultrafiltration membrane having a fractionation
molecular weight of from 3,000 to 15,000 is used. For a membrane
having a fractionation molecular weight of less than 3,000, the
filtration resistance becomes too high and the filtration time
becomes longer. Therefore, this is uneconomical. When the
fractionation molecular weight exceeds 15,000, the filtration
accuracy lowers. The membrane may be made of polysulfone,
polyacrylonitrile, sintered metals, ceramics, or carbon. Any
material may be used, but a membrane made of polysulfone is easy to
use in view of heat resistance and filtration speed. The membrane
may have any shape including spiral, tubular, hollow fiber and the
like, but a hollow fiber membrane is compact in size and easy to
use. Further, when metal impurities are washed out and removed by
the same ultrafiltration process, if necessary, an additional
operation, such as further washing out and removing with adding
pure water even after an aimed concentration is obtained, can be
performed in order to improve the removal rate. In the course of
this process, the silica is concentrated to preferably from 10 to
60 wt %, and particularly from 20 to 50 wt %.
[0057] Still further, before or after the ultrafiltration, if
necessary, purification with ion exchange resins may be added. For
example, by contacting with an H-type strong acid cation exchange
resin, impure metals and alkali metals that contamiante in the
particle-growing process may be removed. Through deanionization
purification performed by contacting with an OH-type strong basic
anion exchange resin, still higher purity may be attained.
[0058] In this way, high-purity colloidal silica having a silica
particle diameter from 10 to 200 nm and a silica concentration from
10 to 60 wt % is obtained.
[0059] Then, the resulting colloidal silica is admixed with a
buffer solution, which is a combination of a weak acid having a pKa
from 8.0 to 12.5 at 25.degree. C. (pKa is a logarithm of the
reciprocal of acid dissociation constant) and a quaternary ammonium
base, to obtain the polishing composition of the present invention.
The amount of the buffer solution admixed is such an amount to make
the pH of the polishing composition from 8 to 11 at 25.degree. C.
and to provide buffer action in the range from pH8 to pH11.
[0060] The polishing composition of the present invention thus
obtained is preferably a water dispersion having a silica
concentration from 2 to 50 wt % with respect to the total amount of
the composition. From the viewpoint of still improving the
polishing performance of the polishing composition, the silica
concentration is desirably from 10 to 25 wt %.
[0061] As mentioned above, in the production method of the
polishing composition of the present invention, colloidal silica
having a silica concentration from 10 to 60 wt % is prepared; and
then the aforementioned buffer solution is added to the colloidal
silica to adjust the pH as well as silica concentration. Further,
in order to adjust the silica concentration and/or conductivity in
the polishing composition of the present invention, an aqueous
solution of the aforementioned salts may be added. In addition, it
is desirable that deionized water and others be optionally added as
appropriate to obtain the polishing composition of the present
invention.
[0062] The polishing process for semiconductor wafers using the
polishing composition of the present invention will be mentioned.
In the case of surface polishing, the surface of a semiconductor
wafer is polished as follows: under a state of the surface to be
polished of the semiconductor wafer being pressed against a
rotatable surface table, both or either of the surface table and
semiconductor wafer are rotated while the polishing composition of
the present invention is quantitatively supplied. The rotatable
surface table has a polishing cloth applied to both or either of
upper and lower faces thereof. A polishing machine is used for this
process. As the polishing cloth, for example, there may be used a
synthetic resin foam or suede-like synthetic leather. As the
polishing machine used in the present invention, there may be
mentioned, for example, SH-24 single side polisher and FAM-20B
double side polisher manufactured by SPEEDFAM Co., Ltd.
[0063] In the case of edge polishing, generally, the edge of a
semiconductor wafer is polished as follows: a work (an object
product to be polished), that is a beveled (chamfered)
semiconductor wafer, is pressed against a polishing member, while
the edge of the wafer is inclined and the wafer is rotated, with
the polishing composition being supplied. The polishing member is
composed of a polishing cloth made of synthetic resin foam,
synthetic leather, nonwoven fabric or the like that is applied on
the surface of a rotatable support. An edge polishing machine is
used for this process. As the edge polishing machine used in the
present invention, there may be mentioned, for example, EP-IV edge
polisher manufactured by SPEEDFAM Co., Ltd. The edge-polishing
machine is equipped with a rotatable support having a polishing
cloth applied on the surface thereof, and a rotatable holder that
can clamp the work and be inclined at a desired angle. Both or
either of the work and the support are rotated, while the edge of
the work clamped by the holder is pressed against the support, with
supplying the polishing composition of the present invention to
mirror-polish the edge of the work. More specifically, polishing is
carried out as: the edge of the rotating work is pressed at a given
angle against the support having a polishing cloth and changing
gradually its position by moving upward or downward; and the
polishing composition of the present invention is dropped to the
portion to be polished. The polishing process of semiconductor
wafers using the polishing composition of the present invention
will be described in detail in the following examples. Note that,
the polishing machine is not limited to the above-mentioned
machines, but any machine that is described in, for example,
Japanese Patent Laid-Open Publication No. 2000-317788, Japanese
Patent Laid-Open Publication No. 2002-36079, and others may be
used.
[0064] A polishing composition for semiconductor wafers of the
present invention and a polishing process using the polishing
composition will be further described in detail with reference to
the following example and comparative example, but it should be
construed that the invention is in no way limited to those
examples.
EXAMPLE
(1) Production Example of Colloidal Silica Raw Material-A
[0065] A dilute soda-silicate having a silica concentrated of 4.5
wt % was prepared by adding 520 kg of JIS No. 3 soda-silicate
(SiO.sub.2: 28.8 wt %, Na.sub.2O: 9.7 wt %, H.sub.2O: 61.5 wt %) to
2810 kg of deionized water and uniformly mixing them. The dilute
soda-silicate was dealkalized by passing it through a 1,000-liter
column of a H-type strong acid cation exchange resin (AMBERLITE
IR120B manufactured by ORGANO Corp.) that was preliminary
regenerated with hydrochloric acid. In this way, 3800 kg of an
active silicic acid having a pH of 2.9 and a silica concentration
of 3.7 wt % were obtained. The Na and K contents based on silica in
the active silicic acid were 80 ppm and 5 ppm, respectively. After
that, colloid particles were grown by the build-up process. To the
part (580 kg) of the active silicic acid thus obtained, a 20 wt %
tetramethylammonium hydroxide aqueous solution was added with
stirring to adjust the pH at 8.7, and then the mixture was kept at
95.degree. C. for 1 hour to prepare seed sol. To the resultant seed
sol, the remaining active silicic acid, 3,220 kg, were added over 6
hours. During the addition, the pH was kept at 10 by adding a 20 wt
% tetramethylammonium hydroxide aqueous solution. The temperature
was also kept at 95.degree. C. After the addition, the resulting
reaction product was aged at 95.degree. C. for 1 hour, and left to
cool. The reaction product was filtered off under pressure using a
hollow fiber ultrafiltration membrane having a fractionation
molecular weight of 6,000 (MICROZA UF Module SIP-1013 manufactured
by ASAHI KASEI Corp.) while the reaction product was circulated
with a liquid circulation pump. In this way, the reaction product
was concentrated to 31 wt % of silica concentration to obtain about
475 kg of colloidal silica. The silica particle diameter of the
colloidal silica was 15 nm, and the Na and K contents based on
silica were 13 ppm and 1.2 ppm, respectively. FIG. 1 shows the TEM
image of the colloidal silica. The silica particles shown in FIG. 1
are a mixture of spherical particles and non-spherical particles
having a "oval" or a "V-formation" in which several spherical
particles linked together. The short axis of the colloidal silica
particles was about 20 nm and the long axis was as large as about
50 nm from the TEM image.
(2) Production Example of Additive-A (Salt Aqueous Solution)
[0066] To 37.5 kg of pure water, 37.5 kg of 95 wt % sulfuric acid
were added to prepare 75 kg of dilute sulfuric acid. To the dilute
sulfuric acid, 265 kg of a 25 wt % of tetramethylammonium hydroxide
aqueous solution were dropped to neutralize at pH7 thereby to
prepare 340 kg of a tetramethylammonium sulfate aqueous solution.
Additive-A is an additive agent to increase the conductivity.
(3) Production Example of Additive-B (Buffer Solution)
[0067] Carbon dioxide gas was blown into 164 kg of a 25 wt %
tetramethylammonium hydroxide aqueous solution under vigorous
agitation to neutralize the aqueous solution at pH8.4 thereby to
obtain 184.2 kg of a 33 wt % tetramethylammonium hydrogen carbonate
aqueous solution. To the foregoing aqueous solution, 149.1 kg of a
25 wt % tetramethylammonium hydroxide aqueous solution were admixed
to prepare 333.3 kg of a mixed tetramethylammonium solution used as
a buffer solution. In additive-B, tetramethylammonium hydrogen
carbonate is a salt composed of a combination of a weak acid of
carbonic acid (pKa=10.33) and a strong base, so that the additive-B
serves as the buffer solution in the present invention.
(4) Preparation of Colloidal Silica with pH Buffer Composition
[0068] To 17 kg of the colloidal silica prepared as described
above, the additive-A and additive-B each were added in an amount
shown in Table 1, and then they were mixed for 24 hours. In this
way, a colloidal silica having a pH buffer action and a silica
concentration of 30 wt % was prepared. The properties of three
kinds of colloidal silica, abbreviated as C-1, C-2 and C-3,
respectively, are shown in Table 1. In Table 1, "Total Na
concentration (ppm/SiO.sub.2)" denotes the sodium concentration
based on silica. Further, in the table, the conductivity "mS/m/1 wt
%-SiO.sub.2" denotes the value obtained by measuring the
conductivity of each colloidal silica using a conductivity meter
and dividing the measured value by silica concentration.
TABLE-US-00001 TABLE 1 C-1 C-2 C-3 Colloidal silica raw material-A
(kg) 17 17 17 Additive-A (kg) 0.05 0.01 0.017 Additive-B (kg) 0.22
0.22 0.33 Average diameter (nm) 15 15 15 Silica concentration (wt
%) 30 30 30 Total Na concentration (ppm/SiO.sub.2) 13 13 13
Conductivity (mS/m/1 wt %-SiO.sub.2) 19 20 26 pH 10.2 10.2 10.3
(5) Polishing Test for Semiconductor Wafer Edge
[0069] The colloidal silica shown in Table 1 was diluted with pure
water to obtain the silica concentration shown in Table 2 below.
The following polishing test was performed using the resulting
dilute colloidal silica. The results are shown in Table 2.
Polishing Test
[0070] In accordance with the method described above, polishing
test was performed using an 8-inch silicon wafer having a poly-Si
film. The wafer edge polishing machine used and polishing
conditions were as follows:
[0071] Polishing machine: EPD-200X edge polisher, manufactured by
SPEEDFAM Co., Ltd.
[0072] Wafer rotating speed: 2,000 rpm,
[0073] Polishing duration: 60 sec/wafer,
[0074] Polishing composition flow rate: 3 L/min,
[0075] Polishing cloth: "SUBA400" (manufactured by NITTA HAAS
Inc.),
[0076] Load: 40 N/unit.
[0077] Ten wafers were polished continuously and the tenth wafer
was subjected to the following evaluation.
Evaluation
[0078] After the edge was polished, pure water was supplied in
place of the polishing composition so as to wash it out. The wafer
was removed from the polishing machine and was subjected to the
brush-scrub cleaning using 1 wt % ammonium aqueous solution and
pure water. After that, the wafer was spin-dried while N.sub.2 gas
was blown. For the wafer thus obtained, the number of particles
having a diameter of 0.15 .mu.m or more adhered to the surface of
the wafer was counted by SEM and a laser scattering surface
diagnosis meter. Further, the presence or absence of haze and pit
on the polished surface and also the presence or absence of
unpolished portion caused by incomplete edge polishing were
evaluated by eye-observation under the illumination of a converging
lamp, and furthermore observed under the light microscope at a
magnification of 800. The observation was conducted over the entire
circumference of the work. Still further, the polishing speed was
estimated from the weight difference of the device wafer before and
after polishing.
(6) Polishing Test for Surface Portion of Semiconductor Wafer
[0079] The colloidal silica shown in Table 1 was diluted with pure
water to the silica concentration shown in Table 3. The following
polishing test was performed using the resulting dilute colloidal
silica. The results are shown in Table 3.
[0080] In accordance with the method described above, polishing
test was performed using, as a silicon wafer, a conductive p-type
8-inch etched silicon wafer that was produced by the CZ method and
had a resistivity of 0.01 .OMEGA.cm and (100) crystal orientation.
The wafer polishing machine used and mirror-polishing conditions
were as follows:
[0081] Polishing machine: SH-24, manufactured by SPEEDFAM Co.,
Ltd.,
[0082] Surface table rotating speed: 70 rpm,
[0083] Pressure plate rotating speed: 50 rpm,
[0084] Polishing cloth: "SUBA400" (manufactured by NITTA HAAS
Inc.),
[0085] Load: 150 g/cm.sup.2,
[0086] Polishing composition flow rate: 80 mL/min,
[0087] Polishing duration: 10 minutes.
Evaluation
[0088] After the surface was polished, pure water was supplied in
place of the polishing composition so as to wash it out. The wafer
was removed from the polishing machine and was subjected to the
brush-scrub cleaning with 1 wt % ammonium aqueous solution and pure
water. After that, the wafer was spin-dried while N.sub.2 gas was
blown. The number of particles having a diameter of 0.15 .mu.m or
more adhered to the surface of the wafer thus obtained was counted
by SEM and a laser scattering surface diagnosis meter. Further, the
presence or absence of haze and pit on the polished surface was
evaluated by eye-observation under the illumination of a converging
lamp. Still further, the polishing speed was estimated from the
weight difference of the device wafer before and after
polishing.
COMPARATIVE EXAMPLE
[0089] To 128 kg of conventional sodium-stabilized colloidal silica
("SILICADOL-40": 40.4 wt % of silica concentration, 18 nm of
average particle diameter, and 4,000 ppm of sodium content), 3333 g
of the aforementioned additive-B were added. The resulting solution
was stirred for 24 hours so as to prepare a colloidal silica having
a pH buffer action, a silica concentration of 39 wt %, and a pH of
10.4 that served as a polishing composition (colloidal silica D-1).
The polishing composition had a conductivity of 691 mS/m. The
conductivity divided by silica concentration was 17.7
mS/m/1%-SiO.sub.2. The polishing composition was subjected to
polishing test similarly to Example. The results are shown in
Tables 2 and 3.
TABLE-US-00002 TABLE 2 D-1 Abrasive C-1 C-1 C-1 C-2 C-2 C-2 C-3 C-3
C-3 (Comp. Ex.) Silica concentration 2 4 6 2 4 6 1 3 5 4 (wt %)
Number of particles left 9 13 11 7 12 14 4 8 9 700 behind on wafer
surface (particles/wafer) Pit and Haze on polished no no no no no
no no no no no surface Unpolished portion no no no no no no no no
no no Polishing speed 6.8 8.0 10.9 7.3 9.1 11.1 7.1 9.2 12.3 12.0
(mg/min)
TABLE-US-00003 TABLE 3 D-1 Abrasive C-1 C-1 C-2 C-2 C-3 C-3 C-3
(Comp. Ex.) Silica concentration 2 4 2 4 1 2 4 4 (wt %) Number of
particles left 4 8 7 7 3 8 11 580 behind on wafer surface
(particles/wafer) Pit and Haze on polished no no no no no no no no
surface Polishing speed 0.21 0.26 0.27 0.30 0.22 0.28 0.33 0.39
(.mu.m/min)
[0090] As is clear from the results shown in Tables 2 and 3, in the
polishing test in which the edge was polished with circulating the
polishing composition (a products of the present invention) free of
sodium, and in the mirror polishing test of the surface, the number
of particles left behind on the polished surface was extremely
small, and both adequate polishing speed and edge surface state
were attained and they were favorable. To the contrary, as shown in
the comparative example, in the case of the polishing composition
used without sodium removal, the number of particles left behind on
the polished surface was large, and thus defective effects on the
semiconductor performance was expected.
* * * * *